Gain controllable image intensification system

ABSTRACT

A system which utilizes a plurality of cascaded image intensifier tubes selectively coupled to a power supply to intensify the image of a scene. The power supply is adaptable for supplying the same or different operating potentials to the tubes. The system also includes means coupled to the power supply for protecting the tubes from damage and for providing improved light gain control of the tubes by controlling the amplitude of the operating potentials as a function of the amplitude of tube photocurrent.

United States Patent 1191 1111 3,851,206 Hansen [4 Nov. 26, 1974 GAINCONTROLLABLE IMAGE 1,280,435 /1968 Germany INTENSIFICATION SYSTEM OTHERPUBLICATIONS [75] Inventor: Jay M. Hansen, Santa Monlca,

Calif. Page 142 from F. Eckart, Elektronenoptische Bild- [73] Assignee:Hughes Aircraft Company, Culver wandler und Rontgenblldverstarker,Leipzlg 1956.

City, Calif.

1 Primary ExaminerMalcolm F. l-Iubler [22] Ffled' June 1969 Attorney,Agent, or FirmW. H. MacAllister, Jr.; D. [2]] Appl. No.: 830,592 0.Dennison [52] U.S. Cl 315/30, 313/96, 315/10 151 1111. c1. H01 j 29/52 7ABSTRACT Field of Search 250/213 213 VT; 315/101 A system which utilizesa plurality of cascaded image /30; 3 1 3/941 96 intensifier tubesselectively coupled to a power supply to intensify the image of a scene.The power supply is I References Clted adaptable for supplying the sameor different operat- UNITED STATES PATENTS ing potentials to the tubes.The system also includes 2,177,360 10/1939 Busse 313/96 x means coupledto the Power pp y for Protecting the 3,383,514 5/1968 DOIOII et al.313/96 x es fr m damage and for providing improved light 3,419,74512/1968 Wenzel 315/10 gain control of the tubes by controlling theamplitude 3, 91,233 /19 0 Man ey 1 5/l of the operating potentials as afunction of the ampli- 3,497,699 2/l970 Pietri et al. 3l3/96 X tude oftube photocurrent FOREIGN PATENTS OR APPLICATIONS 1 Cl 4 D F 017.64711/1956 Germany rawmg van Cf +644" 7a 0 ff -gaav 61V /7 )9 6 2 23 l/j/jiz' 52 3 3/ E 35 7| A144 43 74 5mm X L A1146! i 17 45 5: 59 45 57 QC.//VP(/7' 6 I I] 67 l V0.4 746 a c.

42:. :3 A4 1/ H c yapptf l/Vlf/PA/EI GAIN CONTROLLABLE IMAGEINTENSIFICATION SYSTEM BACKGROUND OF THE INVENTION This inventionrelates to image viewing systems and particularly to an imageintensification system which controls the output image brightness andwhich protects the tubes in the system from damage. The invention hereindescribed was made in the course of or under the Contract No.DAABO7-68CO188 with the United States Army.

Conventional image intensification systems employing image intensifiertubes lack either proper light gain control or adequate protection forthe image intensifier tubes or both. Without proper light gain control,the output image intensity will start dropping or cut off completelywhenever the input image intensity increases, and/or the light gaincontrol position is increased, beyond a certain level of brightness.Without adequate protection for the image intensifier tubes, at leastone of the tubes may be damaged whenever the input image intensityincreases, or the setting of the light gain control is increased, beyonda certain level.

At the present time there are no known image intensification systemswhich not only control the light gain such that the output level ofbrightness always in creases with either an increase in the input imageintensity or in the setting of the light gain control but also protectthe tubes from overload conditions.

SUMMARY OF THE INVENTION Briefly, an improved image intensificationsystem is provided which utilizes a plurality of cascaded imageintensifier tubes, a power supply and means coupled to the power supplyfor sensing tube photocurrent in order to improve the light gaincharacteristics of the system while protecting the tubes from overloadconditions.

It is therefore an object of this invention to provide an improved imageintensification system.

Another object of this invention is to provide an image intensificationsystem having proper light gain control and tube protection.

Another object of this invention is to provide an image intensificationsystem in which the image intensifier tubes are cascaded for light gainand paralleled for reception of the power supply voltage.

Another object of this invention is to provide an image intensificationsystem in which the image intensifier tubes are cascaded for light gainand are respectively coupled to a plurality of taps on the power supplyfor receiving different power supply voltages.

Another object of this invention is to provide an image intensificationsystem which utilizes photocurrent of predetermined tubes to control theoperating potentials applied to all of the tubes.

A further object of this invention is to provide an imageintensification system which will not cut off with an increase in theinput light or light gain control level.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects, features andadvantages of the invention, as well as the invention itself, willbecome more apparent to those skilled in the art in the light of thefollowing detailed description taken in consideration with theaccompanying drawings wherein like reference numerals indicate like orcorresponding parts throughout the several views wherein:

FIG. 1 is a graph that illustrates how the output brightness of an imagevaries as a function of either the brightness control position or inputlight level in relation to the system of the invention.

FIG. 2 is a schematic circuit and block diagram in accordance with oneembodiment of this invention.

FIG. 3 illustrates another configuration of the embodiment of FIG. 2 inaccordance with the principles of the invention.

. FIG. 4 is a schematic circuit and block diagram in accordance with asecond embodiment of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In any imageintensification system the output brightness varies as a function ofeither the voltage applied to the tubes or the level of the input light.FIG. 1 illustrates this functional relationship in the curves 11 and 13.Curve 11 illustrates how, in the prior art, the output brightnessdecreases when either the input brightness increases or the brightnesscontrol position is increased beyond a certain level. The outputbrightness may even decrease until an undesirable cut-off conditionexists in the system. Furthermore, systems in the prior art have beenfound to be very susceptible to tube damage in response to an increasein the level of either the input brightness or the applied voltage.Curve 13, on the other hand, illustrates the output gain characteristicsin accordance with this invention, wherein there is no decrease in thelevel of the output brightness with an increase in the level of theinput brightness or brightness control position.

One mechanization of an image: intensification system for achieving thegain control characteristics illustrated in the curve 13 of FIG. 1 isshown in FIG. 2, which illustrates a schematic circuit and block diagramin accordance with one embodiment of this invention. The image of ascene may be focused by a suitable lens (not shown) onto a fiber plate15 of a first image intensifier tube 17 in order to intensify thebrightness of the image of the scene. The tube 17 is coupled to twocascaded image intensifier tubes 19 and 21 in order to further intensifythe image. With the use of the three cascaded intensifier tubes l7, l9and 21, the light coming in one end of the system leaves the other endof the system much brighter and, therefore, the system can greatlyintensify the image. The illustrated image intensifier tubes l7, l9 and21 respectively utilize input fiber plates 15, 23 and 25 and outputfiber plates 27, 29 and 31. The output fiber plate of one tube may beclamped to the input fiber plate of the following tube in order tominimize the loss between adjacent tubes. Fiber plates, which are wellknown in the field of fiber optics, may be used at each end of each ofthe tubes rather than lenses or direct coupling in order to minimize theloss of light and obtain better image resolution as the light passesfrom one tube to the next. Each fiber plate is made up of a large numberof glass fibers placed parallel to each other and fused together allaround and along the length of each of the fibers in order to make thefiber plate vacuum tight. Each fiber consists of a core of glass of ahigh index of refraction surrounded by a layer of glass of a low indexof refraction to enable the fiber to pick up a portion of the light fromthe image at one end and pass it through to the other end. In addition,each glass fiber should be as small or smaller than the least elementthat is to be resolved in order to avoid a loss of resolution of theimage.

Each of the input fiber plates 15, 23 and 25 is used in the conversionof light energy into electrical energy. For this purpose, the frontsurface may be flat to receive the light from the image and the backsurface may be concave. A thin layer of a photosensitive substance isplaced upon the concave surface of each of the input fiber plates 15, 23and 25 to form the photocathodes 33, 34 and 35, respectively. Each ofthe photocathodes 33, 34 and 35 has the property of emitting electronswhen exposed to light in a direct relationship to the intensity of thelight striking it.

Each of the output fiber plates 27, 29 and 31 is used in the conversionof electrical energy back into light energy. To achieve this result,both of the front and back surfaces of each of the output fiber plates27, 29 and 31 may be flat. The front surfaces of the output fiber plates27, 29 and 31, respectively, have the thin phosphor layers 37, 38 and 39placed thereon. Each of the phosphor layers 37, 38 and 39 has theproperty, when struck by electrons, of glowing or emitting light with anintensity directly proportional to the kinetic energy of the electronsupon impact therewith. The phosphor layers 37, 38 and 39 have thinaluminum anode layers 41, 42 and 43, respectively, coated thereon tomake the phosphor layers function better. The aluminum anodes 41, 42 and43 attract the electrons being emitted from their respectivephotocathodes 33, 34 and 35 and provide direct current return paths forthe electrons. In addition, the surface of each of the aluminum anodesthat faces its respective adjacent phosphor layer has a mirror finishthat reflects any light being emitted from the phosphor layer toward thealuminum anode back to the phosphor layer so that the phosphor layer isfurther brightened.

These coated input and output fiber plates are then selectively sealedto opposite ends of the glass envelopes 45, 46 and 47 of the tubes 17,19 and 21, respectively, by a suitable sealing material such as an alloyof metal conventionally used for glass to metal seals. The air is thenexhausted from each of the enclosed glass envelopes 45, 46 and 47 in aconventional manner as is well known in the art.

In the embodiment of FIG. 2, the image of the scene is projected uponthe flat surface of the fiber plate 15 of the image intensifier tube 17,which may be an 80/25 tube, manufactured by Aerojet Delft as model FFF,and also such other companies as Varian and Westinghouse, for example.Portions of the light from the image pass through corresponding fibersin the fiber plate 15 and strike corresponding portions of thephotocathode 33, causing the photocathode to emit electrons. Thephotocathode 33 is coupled through an output terminal to a source of ahigh negative potential, which may be about l5 kilovolts (KV). Theelectrons being emitted from the photocathode 33 are attracted to thealuminum anode 41 which, in turn, is connected through an outputterminal to a reference potential such as ground. Due to the highpotential existing between the photocathode 33 and the anode 41, theelectrons being emitted from the photocathode 33 attain a high energybefore striking the anode 41. The electrons striking the aluminum anode41 pass through the anode 41 to corresponding incremental areas of theadjacent phosphor layer 37, causing the phosphor layer 37 to glow inbrilliance in proportion to the energy and intensity of the electroncharge striking the corresponding incremental areas on the phosphorlayer 37. After striking the phosphor layer, the electrons drift back tothe grounded aluminum anode 41 and then return to the source of the 15KV potential. Overall, the amount of energy which is transferred to thephosphor 37 to excite the phosphor is equal to the energy given to theelectrons minus the energy lost in passing through the aluminum anode41. The gain of the tube 17 (as well as each of the tubes 19 and 21) isa function of the density of the electron stream from its photocathode33, the amount of attraction exerted on this electron stream by itsanode 41 potential, the loss of energy by the electrons in passingthrough the aluminum anode 41, and the characteristics of the selectedphosphor 37. Thus, electrical energy striking the anode I 41 isconverted into light energy by the phosphor 37. This light energy fromthe phosphor layer 37 is then passed through the fiber plate 27 to theadjacent image intensifier tube 19. In the illustrated arrangement, afocus electrode 49 and a zoom electrode 50 are also included in thestructure of the image intensifier tube 17. The focus electrode 49 iscoupled through an output terminal to a source of a voltage which may bevaried from a positive 600 volts to a negative 900 volts, for example.This variable voltage is used to focus the electron stream being emittedfrom the photocathode 33 for the best output resolution of the tube 17.The zoom electrode 50 is coupled through an output terminal to a sourceof potential which may be varied from a positive 1.5 KV to a positive 15KV in order to vary the range of brightness of the image intensifiertube 17 from approximately to 1,000 times brighter than the brightnessof the input image being projected upon the flat surface of the fiberplate 15. This brightness range of from 100 to 1,000 times depends onthe magnification of the tube 17, which in turn is dependent upon thevoltage on the zoom electrode 50. The magnification, which may bedefined as the size of the output image diameter divided by the size ofthe input image diameter, can have a range of, for example, A; to 1.When the magnification is 54;, the diameter of the photocathode 33 iseffectively three times greater than the diameter of the phosphor 37 andhence the electrons are concentrated on the phosphor 37 to give thebrightness gain of 1,000 times. When the magnification is l, therespective diameters are effectively equal and the brightness gain is100 times. When the voltage applied to the zoom electrode 50 is at itsmost positive potential, 15 KV, the brightness gain will beapproximately 1,000 times and when the voltage on the zoom electrode 50is at its lowest positive potential, 1.5 KV, the brightness gain will beapproximately 100 times.

The output intensified image from the output fiber plate 27 of the imageintensifier tube 17 is applied to the input fiber plate 23 of the imageintensifier tube 19, which may be a 25/25 diode tube manufactured bysuch companies as Machlett, Varo and Westinghouse. The light passesthrough the fibers in the fiber plate 23 and strikes the photocathode 34which also emits electrons in direct proportion to the intensity of thelight striking the photocathode 34. The photoelectrons are attracted toan aluminum anode layer 42. The phosphor layer 38 sandwiched between thealuminum layer 42 and the fibre plate 29 responds to the chargeandintensity of the electrons hitting the aluminum layer 42 and convertsthe electron energy into light energy which, in turn, is coupled throughthe output fiber plate 29 into the input fiber plate 25 of the imageintensifier tube 21 for additional image intensification by the tube 21.The focusing electrodes 51 and 52 are respectively coupled to the anodes42 and 43 of the tubes 19 and 21. Although the diode tubes 19 and 21both brighten the image, neither tube changes its magnification of one(I). The tube type and operation of the image intensifier tube 21 areidentical to those of the image intensifier tube 19. The photocathodes34 and 35 of the image intensifier tubes 19 and 21, respectively, arecoupled through output terminals to a reference potential such asground. The anodes 42 and 43 of the tubes 19 and 21, respectively, arecoupled together and through a resistor 53 to an output terminal 55 of ahigh voltage multiplier 57, which produces an output regulated directcurrent (DC) voltage at the terminal 55. The output fiber plate 31 ofthe image intensifier tube 21 may be viewed directly, may be clamped toan eye piece (not shown) for viewing the image directly at the output ofthe tube 21, may have a fiber bundle (not shown) clamped to it fortransmitting the image to another location, or may be utilized in anydesired arrangement for viewing or sensing the image. A fiber bundle isquite similar to any of the fiber plates previously mentioned and iswell known in the art. The fiber bundle, however, does not have a layeror coating on either end and each fiber in the fiber bundle picks uplight and displays it at the remote end. The fiber bundle may be epoxiedtogether at both ends.

The power supply of the system includes a DC voltage regulator 59, abrightness control unit or potentiometer 61, a DC chopper circuit 65, astep-up transformer 67 and the high voltage (H.V.) multiplier 57. Theoutput regulated DC voltage from the system power supply is produced inthe following manner. A DC input voltage is applied to the DC voltageregulator 59 which has its output level controlled by the setting of thebrightness control potentiometer 61 which, in turn, is coupled between apositive DC voltage and ground. The regulated DC voltage from the DCvoltage regulator 59 is applied to the DC chopper 65 which converts thepure DC voltage into a bipolar square wave output voltage proportionalto the DC voltage input and at a peak to peak (P/P) amplitude of, forexample, 40 volts. This square wave output voltage is applied to theprimary of the step-up transformer 67, which may have a 50:1 voltagestep-up ratio, and the 2,000 volt P/P output from the secondary of thetransformer 67 is applied to the high voltage multiplier circuit 57. Theoutput voltage of the high voltage multiplier 57 may be applied to afilter circuit (not shown) for additional filtering before being appliedthrough the resistor 53 to the anodes 42 and 43 of the tubes 19 and 21,respectively. The voltage regulator 59, DC chopper circuit 65 andtransformer 67 are of conventional types, as are well known in the art.The high voltage multiplier circuit 57 will be explained in more detailin connection with FIG. 4.

For proper tube life, the maximum allowable anode voltage on each of thediodes 19 and 21 in relation to the corresponding photocathode should beapproximately 15,000 volts. With a decrease in this anode voltage therewill be little loss of image resolution but a decrease in the outputgain level. It should, therefore, be obvious that by varying thebrightness control potentiometer 61, the output voltage of the highvoltage multiplier 57, and hence the anode voltages of the tubes 19 and21, will be changed, thereby changing the output gain of the system.

Some of the purposes of the invention, as stated with reference to theembodiment of FIG. 2, are to properly control the light gain of thesystem and protect the tubes 19 and 21 from damage caused by any of suchoverload conditions as an excess light source, voltage or current. Thesebasic purposes are achieved by mechanizing the system, as shown in theembodiments of FIGS. 2 and 4, so that the last tube 21 controls thevoltage on both of the tubes 19 and 21, and the tube 19 is able to becut off before the tube 21.

For proper control of the light gain of the system, it is desirable toraise or lower the voltage on tube 21 as well as on tube 19. When thesevoltages applied to the anodes are reduced, the gains of the tubes 19and 21 are reduced. With the arrangement shown in FIG. 2,

the gain of both tubes is controlled. As previously discussed, the tubes19 and 21 are electrically coupled together in parallel and through theresistor 53 to the terminal 55 in order to receive the same anodepotential. Because the photocurrents of both of the tubes 19 and 21 flowthrough the common resistor 53, which limits the power to the tubes, thevoltages supplied to their respective anodes will be decreased with anincrease in photocurrent, and vice versa. As a result, the gain of bothof the tubes 19 and 21 is affected by the change in their anodevoltages, which enables the system to have a wide range of gain. Sincethe tube 21 has a much larger photocurrent than that of the tube 19 andthe photocurrents of both tubes flow through the common resistor 53, theamplitude of the photocurrent of tube 21 basically controls theamplitudes of the anode voltages on the tubes 19 and 21, therebycontrolling the light gain of the system and protecting both of thesetubes from damage. The system shown in FIG. 2 can never exhibit theoutput brightness characteristics as shown in the curve 11 of FIG. 1but, on the other hand, will produce the output brightnesscharacteristics as shownin the curve 13 of FIG. 1 since, as explainedbelow, the tube 19 is able to be cut off before the tube 21.

The ability of the tube 19 to be cut off before the tube 21 can beachieved, in one mechanization, by selecting the tube 19 so that it hasa higher threshold of cut-off than that of the tube 21. The threshold ofcutoff values of these 25/25 tubes 19 and 21 is nominally between 2.5 KVand 4.5 KV. For example, the tubes might be chosen so that tube 19 cutsoff when its anode potential drops below 4 KV and the tube 21 cuts offwhen its anode potential drops below 3 KV. In this case, since thephotocurrent in the tube 21 is used to control the anode voltage on eachof the tubes 19 and 21 and tube 19 is chosen to cut off before tube 21,neither would be cut off. This conclusion is based upon the fact thattube 19 has its anode voltage controlled basically by the photocurrentin tube 21 and, if there is no photocurrent in tube 21, the tube 21cannot cut off the tube 19.

FIG. 3 shows a modification of the embodiment of FIG. 2 wherein a 25/25diode tube 69, identical with the tubes 19 and 21 of FIG. 2, replacesthe /25 tetrode tube 17 of FIG. 1. Tubes 19 and 21 are shown cascaded toeach other and to this new diode tube 69. The anodes 70, 42 and 43 ofthe tubes 69, 19 and 21, respectively, are connected together at acommon junction point 71 and through the resistor 53 to the output highvoltage terminal 55 of the high voltage multiplier 57 of FIG. 2. Thesethree tubes 69, 19 and 21 are now chosen so that the first tube in thecascaded sequence, tube 69, has the highest threshold of cut-off and thelast tube in the cascaded sequence, tube 21, has the lowest threshold ofcut-off. With this arrangement the system would also produce the outputbrightness characteristics of the curve 13 of FIG. 1. It should beobvious that two or more diodes could be used in this cascaded sequenceto attain the intensified image output. With these three tubes 69, 19and 21 connected in a cascaded sequence, the tube 21 would basicallycontrol the anode voltages on the tubes 69, 19 and 21. The tubearrangement of FIG. 3, as well as that of FIG. 2, would provideprotection for all of the tubes in the system that receive their anodevoltages through the resistor 53, and, as shown in graph 13 of FIG. 1,would cause the output brightness to increase without decreasing as theinput brightness increases or the brightness control 61 position isincreased.

It should be noted at this time that the parallel connections of thetubes to the power supply as shown in FIGS. 2 and 3 will producespurious lights whenever there is a difference of potential across theinterface between the anode of one tube and the photocathode of theadjacent tube or between the anode of tube 21 and a fiber bundle (notshown) connected thereto. These spurious lights, which may be generatedat the interfaces, may not be sufficiently bright to interfere with theoperation of the system even though they may be amplified by a followingtube. However, these spurious lights may be reduced and/or eliminated byutilizing a second embodiment of the invention, as shown in FIG. 4.

Referring now to FIG. 4, an arrangement is illustrated whereby thelight-gain cascaded tubes 19 and 21 have their anodes 42 and 43respectively coupled to taps 73 and 75 of a high voltage (H.V.)multiplier 77. The embodiment shown in FIG. 4 still enables the tube 21to control the voltages on the tubes 19 and 21, affords tube protectionfor the tubes 19 and 21 and enables the output voltages from the taps 73and 75 of the high voltage multiplier 77 to track each other. In theabsence of input light striking the input fiber plate 23 of the tube 19,the photocurrents of the tubes 19 and 21 are at a minimum amplitude andthe voltages at the taps 73 and 75 will be approximately 15 KV and 30KV, respectively. The anode 42 of the tube 19 is connected to thephotocathode 35 of the tube 21 so that there is no difference ofpotential across the interface between the tubes 19 and 21, and hencesubstantially no spurious lights will be generated at that interface.The photocathode 34 of the tube 19 is connected to ground. The type andoperation of the DC voltage regulator 59, the DC chopper circuit 65, andthe transformer 67 are identical to those shown in FIG. 2. The H.V.multiplier 77 is similar to the high voltage multiplier 57 of FIG. 2,but has two output voltage taps 73 and 75 rather than one shown by theoutput terminal 55 of FIG. 2.

As shown in FIG. 4, the control voltage which controls the output of thevoltage regulator 59 is derived by a different circuit than that shownin FIG. 2 in that it is produced by a sensing circuit 78, rather than bythe direct output from the movable arm of a brightness controlpotentiometer 79. The sensing circuit 78 is coupled between the bottomof the secondary winding of the transformer 67 and an upper portion ofthe H.V. multiplier 77 in order tosense the charging current therefrom.The sensing circuit 78 includes: the brightness control potentiometer 79which is coupled be tween a source of positive voltage and ground; abypass capacitor 80 coupled between the movable arm of the potentiometer79 and ground to bypass any noise or ripple to ground; a half-waverectifier comprised of the rectifiers 81 and 82, the sensing resistor83, the resistor 84 and a capacitor and, finally, a coupling capacitor86 which is coupled between the junction of rectifiers 81 and 82 and anupper portion of the H.V. multiplier 77 near the 15 KV tap 73. Thecommon junction of the movable arm of the potentiometer 79, therectifier 81, the resistors 83 and 84, and the capacitors 80 and 85 iscoupled to the bottom of the secondary winding of the transformer 67, asshown in FIG. 4. Because the couplingcapacitor 86 in the sensing circuit78 has one side very close to the 15 KV output tap 73 and the other sidevery close to a ground potential, the coupling capacitor must have a DCvoltage rating of at least 15 KV. The operation of this sensing circuit78 will be explained at a later time.

The 2,000 volt peak-to-peak output from the secondary of the step-uptransformer 67 is applied to a plurality of serially coupled networks87, 89, 91 and 93 which rectify and multiply this 1,000 volt peak outputto produce the first output voltage of 15 KV at the tap 73. A circuit95, which will be explained later, is serially coupled between thenetwork 93 anda sequence of nine more serially coupled networks 97 and99 which develop the second output voltage of 30 KV at the tap 75.

Each of the networks 87, 89, 91, 93, 97 and 99 is comprised ofcapacitors 101 and 102 and the rectifiers 103 and 104 to form oneconventional cascade voltage doubler section in the high voltagemultiplier 77. Each network is sequentially responsive to its respectiveinput voltage to produce a higher output voltage across its respectivecapacitor 102 of approximately 2,000 volts which is serially additive tothe output voltage from the preceding network and/or to the outputvoltage from the following network to obtain an effective voltagemultiplication of the 1,000 volt peak output from the transformer 67.Each of the capacitors 101 and 102 may be selected to have a capacitanceof approximately 1,500 micro-microfarads and a DC working voltage of2,000 volts. Each of the rectifiers 103 and 104 is selected to have aminimum of capacitance. It should be noted at this time that the circuitis identical to any of the networks 87, 89, 91, 93, 97 and 99, exceptfor the omission of the capacitor 102. The anode of the rectifier 103 inthe circuit 95 is coupled to thetap 73, while the cathode of therectifier 104 in the circuit 95 is coupled to the coupling capacitor 86in the sensing circuit 78. The operation of the circuit 95 will beexplained in conjunction with the operation of the sensing circuit 78.

A firt bleeder resistor 107 is connected between the 15 KV tap 73 andthe 30 KV tap 75 and a second bleeder resistor 109 is connected betweenthe 15 KV tap 73 and the bottom side of the secondary winding of thetransformer 67. These resistors 107 and 109 form a bleeder resistornetwork for the RV. amplifier 77 and have relatively high resistances tominimize any loading of the multiplier 77. These bleeder resistors 107and 109 are required to discharge the capacitors 101 and 102 in theserially coupled networks, and thereby reduce the anode voltages of thetubes 19 and 21 when the brightness gain setting of the potentiometer 79is reduced or the system is turned off.

In the operation of the high voltage multiplier, an alternation of onepolarity of the square wave input is applied to the first network 87which causes the serially coupled capacitors 101 on the upper portion oneach of the networks to charge all down the line through theirrespectively forward-biased rectifiers 103. On the next alternation ofthe opposite polarity the charge on the capacitor 101 in any network isseries aiding with the input voltage to that network. This conditionrespectively reverse-biases the rectifier 103 and forwardbiases therectifier 104 in that network, thereby completing the path for thedischarge of the capacitor 101. The discharge of the capacitor 101, inconjunction with the input voltage to the network, charges the capacitor102 in the same network to a higher voltage than the input voltage tothe network. After several cycles of the square wave input, thecapacitors 102 are charged up, thereby achieving a voltagemultiplication.

The sensing circuit 78 senses the charging current resulting from thephotocurrent in the upper part of the high voltage multiplier 77 fromthe tube 21 and uses this to control the amplitude of the output voltagefrom the regulator 59, which in turn controls the amplitude of thevoltage in the secondary of the transformer 67. Due to the intentionalomission in the circuit 95 of the capacitor 102 between the anode of therectifier 103 and the cathode of the rectifier 104, the charging currentto the upper part of the high voltage multiplier 77 is forced to go intothe sensing circuit 78 and passes through the coupling capacitor 86 tothe junction of the rectifiers 81 and 82. Before this charging currentis re turned to the serially coupled networks 97 and 99 in order tocomplete the circuit return back to the secondary of the transformer 67,it is subjected to half wave rectification. Positive going surges inthis charging current are shunted by the rectifier 81 around theresistor 84 to the bottom part of the secondary of the transformer 67.Negative going surges in this charging current are shorted by therectifier 82 and pass through the resistor 83 to the bottom part of thesecondary of the transformer 67. The capacitor 85 is coupled across theresistor 83 to filter the current fluctuations passing through therectifier 82 and produce a relatively smooth DC voltage across theresistor 83. The resistor It improves the operation of the half waverectifier by stabilizing the potential at the junction of the rectifiers01 and 82.

The sensing circuit 78 compensates for the bleeder current in thefollowing manner. The DC voltage drop across the sensing resistor 83 isproportional to the bleeder current through the resistor 107, as well asto the photocurrent from the tube 21. That component of the voltage dropacross the sensing resistor 83 which is proportional to the bleedercurrent through the resistor 107 is proportional to the high voltagedeveloped at the tap 75, which in turn is proportional to the controlvoltage applied from the sensing circuit 78 to the regulator 59. Thecontrol voltage is equal to the voltage selected by the setting of thebrightness control potentiometer 79 minus the voltage developed acrossthe sensing resistor 83. Thus a feedback loop is closed. The basiceffect of that component of the voltage drop across the sensing resistor83 which is caused by the flow of bleeder current through the resistor107 is to increase the range of the control potentiometer 79 that isrequired to control the system. Therefore, this system effectivelycompensates for the bleeder current by subtracting from the voltageproduced by the current sensed by the sensing circuit 78 that part whichis pro duced as a result of bleeder current flow. The difference betweenthe two is due to the photocurrent in the tube 21.

With an increase in the photocurrent of the tube 21 only, the amplitudeof the control voltage applied to the DC voltage regulator 59 willdecrease, thereby causing the output of the DC voltage regulator 59 todecrease. The effective resistance (load line) presented to the tube 21is controlled by the value of the sensing resistor 83. The photocurrentin the tube 19 will not at"- fect the operation of the sensing circuit78.

Image intensification systems in the prior art basically utilize cascadevoltage connections to the power supply similar to those shown in FIG. 4in order to avoid spurious lights. However, the mechanizations of priorart systems, as mentioned before, provide very poor gain controlcharacteristics and tube protection. Conventional arrangements using amultiplier stack cause the voltage on the upper end of thestack (lasttube 21) to drop to low values, causing a characteristic similar to thatof the curve 11 of FIG. 1. The use of series load resistors withcascaded voltage connections also leads to the characteristic shown bythe curve 11 of FIG. 1, with the output being completely cut off, andthe first tube being overloaded both in voltage and in power.

The tube protection furnished by the embodiments shown in FIGS. 2, 3 and4 take into consideration the maximum amount of power that would bedelivered to any of the tubes. More specifically, in case of a constantpower supply resistance the maximum transfer of power from the source tothe tube occurs when the voltage applied across the tube is equal toone-half of the supply voltage. This maximum power occurs, there fore,when the power applied to the tube equals onefourth of the square of thevoltage applied to the resistor connected between the power sourcevoltage and the anode of the tube divided by the resistance of theresistor. In the embodiments shown in FIGS. 2, 3 and 4 the circuits areso arranged such that at this voltage the maximum power is notexcessive. This affords good protection to the tubes provided the spotshave sufficient size.

It should be noted that, with the arrangement shown in FIG. 4, it is notnecessary to select the tubes 19 and 21 according to their threshold butthe power supply could be arranged such that tube 19 would cut offbefore tube 21. For example, the voltages at the taps 73 and could be 13KV and 28 KV, respectively, so that the tube 19 would operate at a loweranode-to-cathode potential than the tube 21. It should be further notedthat instead of the anode of tube 19 being at the same potential as thephotocathode of tube 21, the system could be arranged so that there isan overlapping of potentials. For example, the 15 KV terminal could havebeen connected only to the anode of tube 19 while the photocathode andthe anode of tube 21 could have been respectively connected to, forexample, and 25 KV taps on the power supply. This would produce anoverlapping voltage arrangement such that the potentem which protectsthe tubes from damage, and provides good gain control or outputbrightness characteristics such that the tube anode voltages from thepower supply track each other.

While the salient features have been illustrated and described withrespect to two particular embodiments, it should be readily apparentthat modifications can be made within the spirit and scope of theinvention as set forth in the appended claims.

What is claimed is:

l. A system for intensifying an image comprising:

a plurality of intensifier means cascade coupled to one another insequence for respectively increasing the brightness of an image of ascene, each of said plurality of intensifier means operating to developan output image of the scene at a more intensified brightness level thanthat presented thereto, each of said intensifier means having theability to be cut off before the next succeeding intensifier means insaid sequence;

power supply means coupled to each of said plurality of intensifiermeans for simultaneously applying variable operating potentials at otherthan zero potential to said plurality of intensifier means; and

sensing means, coupled to said power supply means, being substantiallyresponsive to the amplitude of the photocurrent in the last intensifiermeans in said sequence for causing the operating potentials to be variedas a function thereof in order to control the gain of said plurality ofintensifier means and the brightness of the output image of the scene asa function of the amplitude of the photocurrent being sensed.

2. The system of claim 1 wherein:

each of said plurality of intensifier means is an image intensifiertube, each of said image intensifier tubes being selected to have ahigher threshold of cut-off characteristic than the following imageintensifier tube connected in cascade therewith.

3. The system of claim 1 further including:

an amplifying tube coupled to said plurality of intensifier means, saidamplifying tube being responsive to the reception of the image of thescene for brightening the image of the scene and applying the brightenedimage of the scene to said plurality of intensifier means; and I secondmeans coupled to said amplifying tube for supplying operating voltagesthereto.

4. The system of claim 3 wherein:

said amplifying tube is a tetrode intensifier tube.

'5. The system of claim .1 wherein:

said power supply means includes output means coupled to each of saidplurality of intensifier means for simultaneously providing the sameoperating potentials thereto; and

said sensing means includes a resistor coupled between said output meansand each of said plurality of intensifier means and being responsive toa change in the total amplitude of the photocurrents from said pluralityof intensifier means for inversely changing the amplitude of theoperating potentialssimultaneously being applied to said plurality ofintensifier means.

6. The system of claim 5 wherein:

said plurality of intensifier means includes first and secondintensifiertubes.

7. The system of claim 1 wherein:

said power supply means includes a tapped high voltage output circuitfor respectively providing a different operating potential to each ofsaid plurality of intensifier means.

8. A system for intensifying an image comprising:

a plurality of intensifier means cascade coupled to one another insequence for respectively increasing the brightness of an image of ascene, each of said intensifier means having the ability to be cut offbefore the next succeeding intensifier means in said sequence;

power supply means selectively coupled to each of said plurality ofintensifier means for applying operating potentials to said plurality ofintensifier means, said power supply means including a tapped highvoltage output circuit for respectively provid ing a different operatingpotential to each of said plurality of intensifier means; and

sensing means coupled to said power supply means for selectively sensingthe photocurrent of said plurality of intensifier means, said sensingmeans being responsive to the amplitude of the photocurrent in the lastintensifier means in said sequence to vary the operating potentials tocontrol the gain of said plurality of intensifier means and thebrightness of the output image of the scene as a function of thephotocurrent being sensed, said sensing means including a first circuitcoupled to a tap in the tapped high voltage output circuit forregulating the ampli tude of the voltage applied to said tapped highvoltage output circuit as a function of the amplitude of thephotocurrent being sensed by said sensing means.

9. The system of claim 8 wherein said power supply means includes:

a voltage regulator being responsive to the reception of an input directcurrent voltage and a control signal from said first circuit in saidsensing means for producing an output direct current voltage having anamplitude which varies as a function of the amplitude of thephotocurrent sensed by said sensing means;

a chopper circuit coupled to said regulator and being responsive to theoutput voltage therefrom for producing a direct current square waveoutput voltage having an amplitude proportional to the amplitude of theoutput voltage of said regulator;

a transformer coupled between said chopper circuit and said tapped highvoltage output circuit for transferring the direct current square waveoutput voltage to said tapped high voltage output circuit, said tappedhigh voltage output circuit being responsive to the square wave outputvoltage for developing the different operating potentials; and

control means coupled to said tapped high voltage output circuit forvarying the operating potentials to each of said plurality ofintensifier means in order to control the brightness of the outputintening different potentials along the sequence. Slfied mage of thescene 11. The system of claim wherein:

10. The system of claim 9 wherein: l f l d d said tapped high voltageoutput circuit is a high volt- Sal p um lty O mtensl means me u es anage multiplier having a plurality of networks con- 5 Second imageintensifier tubesnected in sequence with one another for develop-

1. A system for intensifying an image comprising: a plurality ofintensifier means cascade coupled to one another in sequence forrespectively increasing the brightness of an image of a scene, each ofsaid plurality of intensifier means operating to develop an output imageof the scene at a more intensified brightness level than that presentedthereto, each of said intensifier means having the ability to be cut offbefore the next succeeding intensifier means in said sequence; powersupply means coupled to each of said plurality of intensifier means forsimultaneously applying variable operating potentials at other than zeropotential to said plurality of intensifier means; and sensing means,coupled to said power supply means, being substantially responsive tothe amplitude of the photocurrent in the last intensifier means in saidsequence for causing the operating potentials to be varied as a functionthereof in order to control the gain of said plurality of intensifiermeans and the brightness of the output image of the scene as a functionof the amplitude of the photocurrent being sensed.
 2. The system ofclaim 1 wherein: each of said plurality of intensifier means is an imageintensifier tube, each of said image intensifier tubes being selected tohave a higher threshold of cut-off characteristic than the followingimage intensifier tube connected in cascade therewith.
 3. The system ofclaim 1 further including: an amplifying tube coupled to said pluralityof intensifier means, said amplifying tube being responsive to thereception of the image of the scene for brightening the image of thescene and applying the brightened image of the scene to said pluralityof intensifier means; and second means coupled to said amplifying tubefor supplying operating voltages thereto.
 4. The system of claim 3wherein: said amplifying tube is a tetrode intensifier tube.
 5. Thesystem of claim 1 wherein: said power supply means includes output meanscoupled to each of said plurality of intensifier means forsimultaneously providing the same operating potentials thereto; and saidsensing means includes a resistor coupled between said output means andeach of said plurality of intensifier means and being responsive to achange in the total amplitude of the photocurrents from said pluralityof intensifier means for inversely changing the amplitude of theoperating potentials simultaneously being applied to said plurality ofintensifier meaNs.
 6. The system of claim 5 wherein: said plurality ofintensifier means includes first and second intensifier tubes.
 7. Thesystem of claim 1 wherein: said power supply means includes a tappedhigh voltage output circuit for respectively providing a differentoperating potential to each of said plurality of intensifier means.
 8. Asystem for intensifying an image comprising: a plurality of intensifiermeans cascade coupled to one another in sequence for respectivelyincreasing the brightness of an image of a scene, each of saidintensifier means having the ability to be cut off before the nextsucceeding intensifier means in said sequence; power supply meansselectively coupled to each of said plurality of intensifier means forapplying operating potentials to said plurality of intensifier means,said power supply means including a tapped high voltage output circuitfor respectively providing a different operating potential to each ofsaid plurality of intensifier means; and sensing means coupled to saidpower supply means for selectively sensing the photocurrent of saidplurality of intensifier means, said sensing means being responsive tothe amplitude of the photocurrent in the last intensifier means in saidsequence to vary the operating potentials to control the gain of saidplurality of intensifier means and the brightness of the output image ofthe scene as a function of the photocurrent being sensed, said sensingmeans including a first circuit coupled to a tap in the tapped highvoltage output circuit for regulating the amplitude of the voltageapplied to said tapped high voltage output circuit as a function of theamplitude of the photocurrent being sensed by said sensing means.
 9. Thesystem of claim 8 wherein said power supply means includes: a voltageregulator being responsive to the reception of an input direct currentvoltage and a control signal from said first circuit in said sensingmeans for producing an output direct current voltage having an amplitudewhich varies as a function of the amplitude of the photocurrent sensedby said sensing means; a chopper circuit coupled to said regulator andbeing responsive to the output voltage therefrom for producing a directcurrent square wave output voltage having an amplitude proportional tothe amplitude of the output voltage of said regulator; a transformercoupled between said chopper circuit and said tapped high voltage outputcircuit for transferring the direct current square wave output voltageto said tapped high voltage output circuit, said tapped high voltageoutput circuit being responsive to the square wave output voltage fordeveloping the different operating potentials; and control means coupledto said tapped high voltage output circuit for varying the operatingpotentials to each of said plurality of intensifier means in order tocontrol the brightness of the output intensified image of the scene. 10.The system of claim 9 wherein: said tapped high voltage output circuitis a high voltage multiplier having a plurality of networks connected insequence with one another for developing different potentials along thesequence.
 11. The system of claim 10 wherein: said plurality ofintensifier means includes first and second image intensifier tubes.